Petrology of silicic and syenitic facies of the Organ Needle Pluton, Organ Mountains batholith, Dona Ana County, southern New Mexico

Abstract

The early Oligocene Organ batholith of the Organ Mountains volcano-plutonic complex is a rare example of a large composite batholith which has been west-tilted and exposed in structural profile by late Cenozoic faulting. The Organ Needle pluton (ONP) represents_the oldest and largest exposed volume of three comagmatic intrusive_phases in the complex. Present detailed modal, petrographic and_whole-rock chemical data shows trends within the ONP to be consistent_with a hypothesis proposed by previous researchers; that the pluton_has vertical and horizontal chemical and compositional zonations (now_zoned west to east). Exposures of the ONP in the Organ Mountains show_more silicic rocks at the western margin and higher elevations, e.g., alkali feldspar granite (AFG)-quartz-alkali feldspar syenite (QAFS)-quartz syenite, and more intermediate rocks along the eastern margin, e.g., quartz syenite-quartz monzonite-syenite. Samples from four geochemical transects in the Organ Mountains were collected along a 20-km N-S trend in order to test this hypothesis and to establish the extent and continuity of zoning in the ONP. Results show that the pluton is chemically zoned from 73%-77% SiO (sub 2) at the roof, representing the original silicic cap to 61%-71% SiO (sub 2) in the structural interior. The interior of the pluton is zoned from 61% SiO (sub 2) at the base to 71% SiO (sub 2) near the silicic cap. Additionally, the south-central mountain range hosts a frozen mafic diapir which represents the pluton's less-differentiated core; SiO (sub 2) values of these monzodiodtic rocks ranges from 55% to 60%. Field data, chemical and petrological trends support models of assimilation/fractional crystallization (AFC) and magma mixing as mechanisms of zonation. Petrographic analysis indicates that alteration, although not intense, is widespread and appears more prominent toward the pluton roof and margins. Data also suggest that rocks of the ONP show both subsolidus and hydrothermal alteration. Preliminary delta (super 18) O data show that comagmatic volcanic rocks show normal (unaltered) values but certain areas of the pluton have been isotopically modified (depleted in delta (super 18) O & delta D with respect to values of unaltered granitic rock). The lack of a well-established porphyry-type fracture system in the pluton suggests that mechanisms of isotopic modification, if the result of circulating hydrothermal fluids, required some other means of introduction to the system.

52 samples from this reference